Frequency stabilization of microwave oscillations



Aug. 2, 1955 1 E. NORTON 2,714,662

FREQUENCY STAEILIZATION 0E MICROWAVE osCLLATIoNs Filed May 29, 195o @/93 CEL L 75 252 FREQUE/vay nu I INVENTOR ATTORN EY 2,714,662 Patented Aug. 2,1955.

FREQUENCY STABILIZATION F MICROWAVE OSCILLATIONS Lowell E. Norton, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 29, 1950, Serial No. 164,977 10 Claims. (Cl. Z50-36) This invention relates to methods and systems utilizing molecular resonance of gases for stabilization of the frequency of microwave oscillations.

The present invention is an improvement upon that disclosed in copending application Serial No. 5563 in which a modulated Stark effect is used for frequency stabilization.

In accordance with the present invention, the microwave oscillations are impressed upon two confined bodies of gas which in the absence of Stark fields exhibit sharp molecular resonance at a frequency or frequencies substantially displaced from the desired frequency of the microwave oscillations. To provide an errorsignal of polarity reversing with change in sense of the deviation of the oscillations from the desired frequency, there are produced, in the bodies of gas, Stark fields having directf current components which shift the molecular resonances or absorptions to frequencies respectively higher and lower `than the desired frequency of the oscillations and having pulsating components of opposite phase effective in unison to sweep the molecular absorptions toward and from the aforesaid desired frequency. The microwave energies transmitted by the bodies of gas so subjected to Stark fields are demodulated and thence combined, as in an amplitude or phase-comparator network, to produce an error-signal for use in control of frequency of the oscillations. A

The invention further resides in methods and systems having the features of novelty and utility hereinafter described.

For a more detailed understanding of the invention, reference is made to the accompanying drawing in which:

Fig. l is a block diagram of a frequency control System using the invention;

Figs. 2A, 2B, and 3A-3C are explanatory figures referred to in discussion of Figs. l, 4 and 5;

Fig. 4 is a block diagram of a modification of Fig. l; and

Fig. 5 is a schematic diagram of components suited for use in the system of Fig. 4.

Referring to Fig. l, the oscillator may be a klystron, magnetron or other generator of microwave oscillations whose frequency is to be stabilized; alternatively, it may be a medium-frequency oscillator followed by a series of harmonic generators for producing microwave oscillations. In either event, the microwave energy is impressed, as by transmission lines 12A, 12B of waveguide or concentric line type, upon two enclosures 11A, 11B containing gas, such as ammonia, which exhibits sharp molecular resonance at microwave frequencies. Various examples of suitable gas and frequencies at which they exhibit molecular absorption are mentioned in copending applications including Serial No, 1240.

Each of the gas cells may be a chamber or length of waveguide having windows material transparent to the can be sealed to the Walls seal for enclosure of the 9, 9 of quartz, mica or other microwave energy and which of the cell to form a gas-tight gas at suitably low pressures,

Cil

for example of the order of 0.01 millimeter of mercury.

In general, assuming an absorption line of a gas to occur at a frequency fo, the frequency f to which the maximum absorption is displaced upon subjection of the gas to a Stark field having the value e may be expressed as 1) f=f0|2 where a constant gas and line. l If the Stark field has both a D. C. and sinusoidal component, the voltage e applied to the Stark electrode may be defined as (2) e=E (l-l-m sin pt) where m=modulation factor pt=tme-phase angle E=magnitude of D. C. field The displaced frequency at'which maximum absorption occurs at any instant would therefore be expressed ast- If the modulation factor be made small, i. e., that m 1, then the frequency of the shifted absorption may be simply expressed as (4) fJO-l-/SEA2 (l{2m sin pt) When the alternating or pulsating component of the Stark field passes through zero, will be at its displaced mean value, expressed as At the maximum the modulation, the are respectively lf the zero or quiescent value of the displaced absorption be denoted as (S) f1=f0lE2 corresponding absorption frequencies microwave oscillations is frequency fk, there may be' 20A within the gas cell applied to the Stark electrode 11A a direct-current voltage such as derived from battery shift the absorption frequency to the frequency foej-'EiZ This displaced line LA, as appears in Fig. 2A, 'is somewhat higher in frequency than the desired frequency-'fk of the oscillations.

For simplicity of explanation, it will be assumed `that the same gas is used in the second enclosure 11B, and consequently has an absorption line L2 als-o at the frequency fu. The direct-current component of the voltage applied to the Stark electrode value bEi (where b is a constant later defined) so that this absorption line, as shown by line LB, Fig. 2B, is displaced to lesser amount. The frequency jfu-f-bzEiz of the displaced absorption line La is somewhat lower than the desired frequency fk of the microwave oscillations. The same source of direct current 21 may be used for supplying the biasing potential 20B or different sources may be relative magnitudes of the two applied to the Starkelectrodes are selected or adjusted so that absorption lines Li, L2 of the two bodies of gas, as

used. In any event, the

appears in Figs. 2A and 2B, are displaced from their.

original frequencies, whether the same or different, to fredepending upon the particular the absorption frequency positive and negative peak values of 20B is of suitably lower4 of Stark electrode 20A,'

D. C. biasing voltages- The Starkelectrodes 20A, 20B are also suitably con- -nected or coupled to a source of alternating or pulsating voltage, such as a low-frequency oscillator exemplified by block 22 of Fig. l, whose output wave form may be sinusoidal, triangular or any other desired shape. Thus, the shifted absorption line LA of gas in cell 11A is periodically swept over the range i2rn/8E12 and the shifted absorption line of gas in cell 11B is swept at the same frequency or repetition rate over the range i2m1b2E12. The range 2mE12 should be less than (fo-i-Ei'e) fk and range 2mib2E12 should be less than fk-Uo-l-b2E12). If the sweep interval is made greater than this, then in the case of the amplitude comparison detector arrangement of Fig. l detectors 13 and 14 will each produce a voltage due to the maximum of the absorption envelopes and circuit `operation will fail. The corresponding phase or coincidence detector method of Fig. 4 will not fail for this condition of extended sweep. The connection from one of the Stark electrodes to the source of modulating potential includes a 180 phase-shifter 23, of any suitable type, which insures that pulsating components of theStark fields vary in phase opposition as shown by Figs. 3B, 3C so that the sweeping Stark lines in unison move toward and from the desired frequency fk of the u generated oscillations.

` The connections from the modulator 22, or equivalent, includes a proportioning network, exemplified by a potentiometer 37 for adjusting the relationship of the modulation factors m, m1 for the two Stark fields. For the preferred case where the desired frequency fk of the oscillations is midway between the displaced Stark line frequencies (fo-l-Eiz) and (fo-l-IJ2E12), the ratio of the modulation factors should be b2=n mi Capacitors 25, 25 are blocking condensers, used when necessary, to preclude iiow of direct current from source 21 in the modulator or phase-shifter networks.

Assuming the output of the modulator 22 is sinusoidal (curve M of Fig. 3A); during each cycle of the modulation, the molecular absorptions or resonances of the two gas cells 11A and 11B as shown by curves A and B of Figs. 3B, 3C, sweep in unison from frequencies higher t and lower than the desired frequency fk in unison toward and from the desired frequency fk. The modulation factors m, m1 are so adjusted that each of the Stark lines LA, LB moves to the desired operating frequency because if the` sweep ranges are less than indicated in Fig. l, there is adead zone centered on frequency fr; in which the oscillations may vary in frequency without, as hereinafter explained, producing an error signal. The sweep range should not be substantially greater than indicated in Figs. 2A, 2B as has been explained.

The microwave energies respectively transmitted by the two gas cells whose Stark fields are modulation as above described; are respectively impressed upon demodulatorsexemplified in Fig. l by the diodes 13 and 14 which may be replaced by crystal rectiiiers or other non-linear resistances. In each cycle of the Stark field modulation, theoutput of each of the demodulators includes a pulse occurring as the sweeping Stark line passes through the frequency of the microwave oscillations being generated. When the frequency of the oscillations vcorresponds with the desired frequency fk, the pulses of each pair are of the same magnitude and the output of a differential detector network 15 upon which the impulses are impressed is essentially zero. When the frequency of the generated oscillations is higher or lower than the desired frequency,

the polarity of the output of the comparator network is in each case of corresponding sense, reversing as the frequency deviation changes algebraic sign.

In the particular arrangement shown in Fig. l, the cornparator network 15 is of type similar to that disclosed in copending application Serial No. 596,242 which compares the relative amplitudes of each pair of pulses to produce an output whose polarity depends upon the sense of deviation of the frequency of the oscillations from the desired frequency fk and of magnitude dependent upon the extent of the deviation. Specifically, the demodulators 13 and i4 are connected in opposition in series with two pairs of resistances 16-16 and 17-17, the output or error signal ec appearing between the common terminals of resistors 17, 17 and 16, 16. The differential output of the two demodulators is impressed upon an integrating or smoothing network comprising resistor 18 and capacitor 19 so that the error signal for any given frequency deviation is of steady value rather than pulsating.

The sense and magnitude of the error voltage ec may be determined by impressing itupon a vacuum tube voltmeter S, or equivalent, and an operator may adjust a frequency control 26 of the oscillator in correction for the observed frequency-error. Preferably, however, the control voltage is applied to a frequency-control electrode of the oscillator tube or associated control tube automatically to stabilize the oscillator frequency. One specific arrangement for so automatically controlling the frequency of a microwave oscillator is herein later described.

The arrangement shown in Fig. 4 is similar to that of Fig. l except that the comparator network 15A is of type which compares the phase of the output pulses of the demodulators respectively associated with the Stark modulated gas cells 11A, 11B. `Jarious forms of such phase comparator or coincidence detectors are disclosed in copending applications Serial Nos. 4497, now Patent No. 2,702,351 and 29,836, now abandoned. Specifically, as by a pair of rectifiers 13, 13, the microwave energy transmitted by cell 11B is converted to concurrent pairs of oppositely poled pulses applied to one input channel of the phase-comparator. Alternatively, and as shown in the aforesaid applications, a single rectifier may be used and a push-pull amplifier stage interposed between the demodulator and the comparator to provide the oppositely phased pulse output. The other channel, including the dernodulator 14, supplies pulses of one polarity to another input circuit of the comparator. The phase relation between the pulses of the two channels depends upon the existing deviation from fk of the frequency of the oscillations transmitted to the two Starkmodulated gas cells and reverses with reversal in sign of the frequency deviation.

The phase-comparator 15B specifically shown in Fig. 5 may be used with either of the aforesaid arrangements. It comprises a pair of diodes 27, 27, or equivalent nonlinear resistances which are connected in series and oppositely poled, their differential output current traversing the resistances 28, 2S in series with them. The errorvoltage ec, as indicated, is developed between the common terminal of resistors 28, 28 and the common terminal between the anode of one diode and the cathode of the other.

The push-pull input circuit of the comparator upon which the paired impulses of one channel are impressedv comprises a pair of resistances 30, 30 connected in series with each other and in shunt to the resistors 28, 28, the condensers 29, 29 serving to block the direct current produced by the diodes. The `input circuit for the Second channel includes the resistor 31 connected between the t common terminal of the resistors 30, 30 and ground or equivalent return point to apply each of these pulses to the diodes in phase.

There is produced by the phase-comparator the unidirectional signal voltage ee depending in sense and magnitude upon the phase relations between the pulses respectively applied to the two input circuits of the com parator. This signal voltage, smoothed by the integrating network 18, 19, 19A, may be applied directly to vary the potential of a frequency-control electrode of the control tube and the klystron, so that the potential of the anode of the klystron depends upon the potential of the signal grid of the control tube. Thus, any deviation in frequency of the generated oscillations from the frequency fk, Figs. 2A, 2B, produces an error-voltage es which is applied in correction of that deviation.

The initial adjustment of the operating frequency of the klystron may be effected by adjustment of the potentiometer 36 in the grid circuit of the control tube or in other known manner including adjustment of resonant cavity dimensions of the klystron.

It shall be understood the invention is not limited to the specific systems above described and that changes and modilications may be made within the scope of the appended claims.

What is claimed is:

l. A control circuit for use in a system for controlling the frequency of microwave oscillations, said circuit comprising two confined bodies of gas each exhibiting sharp molecular absorption at a frequency displaced from the desired frequency of said oscillations, means for irnpressing said oscillations on said bodies of gas, means for producing in said bodies of gas direct-current Stark fields which displace the sharp molecular absorptions of said bodies of gas to frequencies respectively higher and lower than said desired frequency of the microwave oscillations, means for producing in said bodies of gas pulsating Stark fields of like frequency and opposite phase repeatedly and in unison to sweep said molecular absorptions from said higher and lower frequencies toward and from said desired frequency, means for demodulating the microwave energies respectively transmitted by said bodies of gas, and means for combining the demodulated energies to produce a signal of sense corresponding with the sense of deviation from said desired frequency of the microwave oscillations.

2. A circuit as in claim 1 in which the demodulated energies are so combined that said signal depends upon their relative amplitude.

3. A circuit as in claim l in which the demodulated energies are so combined that said signal depends upon their phase relation.

4. A system as in claim 1 including means for applying the signal to regulate the frequency of a microwave generator producing said microwave oscillations.

5. A circuit as in claim 1 in which the pulsating Stark fields are of 6. A control circuit for use in a system for controlling the frequency of microwave oscillations, said circuit comprising two enclosures each containing gas exhibiting sharp molecular absorption at a frequency displaced from the desired frequency of said oscillations, Stark electrodes respectively disposed in said enclosures, means for applying to each of said Stark electrodes a potential having a direct-current component and a pulsating component, said direct-current components shifting said molecular absorptions to frequencies respectively higher and lower quency.

7. A control circuit for use in a system for controlling the frequency of microwave oscillations, said circuit comprising two enclosures each containing gas exhibiting sorptions to frequencies respectively higher and lower said desired frequency. 8. A circuit as in claim 7 in which the non-linear resistances are oppositely poled and in which their output produce a unidirectional signal dependent in polarity and magnitude upon the differential amplitude of said output currents.

9. A circuit as in claim 7 in which means is provided to convert one of the demodulated energies to push-pull pulses and in which the network is a phase-comparator for producing a unidirectional signal dependent in polarity and magnitude upon the phase relation of said push-pull pulses to the other demodulated energy.

lO. A control circuit for use in a system for stabilizing the frequency of microwave oscillations, said circuit in cluding an error detector, two enclosures each containing gas exhibiting sharp molecular absorption at a frequency displaced from the desired frequency of said oscillations, means for impressing the oscillations upon the gas in said enclosures and for impressing the microwave energies transmitted by the gas to said error detector, Stark electrodes respectively disposed in said enclosures, and means for applying to each of said Stark electrodes a potential having a direct-current component and a pulsating component, said direct-current components shifting said molecular absorptions to frequencies respectively higher and lower than said desired frequency and said pulsating components being oppositely phased to swing said shifted molecular absorptions in unison toward and from said desired frequency.

References Cited in the tile of this patent UNITED STATES PATENTS 

